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Aviation History
1959
1959 - 3173.PDF
FLIGHT, 27 November 1959 FOLLAND GNAT TRAINER A Two-seat Gnat Development for the R.A.F. FLYING training^ in any air force, is precariously balancedbetween an ever-increasing operational requirement forexperience and skill, a more or less constant level of aptitude in the "raw material," and an inevitably economy-conscious airstaff. An increasingly high level of skill is required at entry into the operational squadrons despite the fact that training equip-ment is difficult to modernize and training time has always to be held to a reasonable minimum. From a detailed study of flying training, Folland Aircraft Ltd.have come to the conclusion that the time has come to increase the amount of flying during training and to improve the performanceof the advanced trainer into the supersonic region. Advocating an increase in R.A.F. training time from 280 hr to 350 hr, thecompany has noted that the U.S.A.F. has found it necessary to give 345 hr total training in three stages, but expects to increasethis training to 450 hr in the near future in order to prepare pilots for supersonic operational aircraft. The increase will involvea change from the Mentor, T-28, T-33, F-100 sequence to what might be termed a three-and-a-half stage sequence with a slightlylonger T-34 stage preceding T-37, T-38 and F-100 stages. The R.A.F. equivalent envisaged by Folland consists of a Jet Provostbasic stage of 150 hr followed by an equal stage on Gnat Trainer and a final 80 hr stage on Lightnings. This compares with thepiston Provost, Vampire Trainer and Vampire/Hunter sequence which is now being modified by introduction of the Tet Provost.It is clear that if the overall training time is to yield the required skill it must be increased, and the advanced trainer must havethe right performance characteristics to form an effective stepping stone between the viceless, simple basic type and the final stageon a two-seat version of the operational intercepter type. Service finance branches usually prefer to economize by con-tinuing the use of the older trainers and compensating for the lower skill normally developed by increasing the time each traineespends flying them. Yet a potentially dangerous situation can arise where the step between an out-moded trainer and the opera-tional type is so great that no amount of flying in the first will effectively prepare the pupil for the second. The aim in any casemust be to achieve at each stage the highest possible standard of skill and this in itself will produce economies in terms of flyinghours required. Perhaps the answer really is that penny-wise economy in flying training does not necessarily pay off in thecritical element of pilot skill. It is satisfying to note that over the last decade, the R.A.F.'schoice of trainers for the various stages has proved to be sound in relation to the performance of the operational type. More recentlythe R.A.F. became the first Service in the world to standardize a jet aircraft tor ab initio basic training. Not too long after theMach 2 Lightning enters service, we may look forward to a train- ing syllabus based on the Jet Provost, Gnat Trainer and LightningT.4. The Gnat fighter can be regarded as having a performancegenerally parallel to that of the Hunter F.6 and Folland are now completing Indian, Finnish and other orders for a total of 59 ofthese aircraft. A detailed description and cut-away drawing of the Gnat Mk 1 appeared in Flight for November 22, 1957. In 1957,Folland also proposed a tandem two-s;at version of the Gnat as a trainer and, during subsequent years, the R.A.F. monitored itsdevelopment and finally placed an order. The trainer was to differ relatively little from the single-seater and would maintain thelatter's high-speed performance while providing the special low- speed and landing characteristics required for training and opera-tion in all practicable weather conditions from a 2,000 yd runway with I.L.S. It could reach the "class room" height of 40,000ft ina fraction over seven minutes, could withstand 6g with drop-tanks fitted at M 1.2, attain M 0.97 in level flight at the tropopause andbe cleared for two to three turns of a spin each way with drop- tanks in place. In order to meet take-off and landing requirements, a largerwing has been developed with an area, at 175 sq ft, nearly 40 sq ft greater than that of the Mk 1. To offset increased drag, the t/cratio has been reduced from 8 to 7 per cent. The R.A.E. 102 modified wing section has been retained, but the aspect ratio is3.29:1 and the centre-line chord 9ft 9in. Anhedra! is 5 deg and incidence is nil. Sweepback is 40 deg at 25 per cent chord. It was realized that the simplest way of increasing the lift ofthe Gnat wing was to extend its area; and this, coupled with the addition of inboard plain flaps, produced a 40 per cent increase inlift and a 10 per cent improvement of CL.nax. The flaps provided greater drag and a consequent reduction in minimum drag-speedwhich has in practice resulted in a more coTifortable approach, particularly for I.L.S. flying. By virtue of the reduction in t/cratio from 8 to 7 per cent, the drag of the larger Trainer wing at speeds below M 1 is within one per cent of that of the fighterwing. Outboard ailerons have provided greater lateral control at low speeds. The Gnat fighter is not deficient in this respect, butthe new arrangement is particularly suited to carrier-landing requirements. Incidentally, manual control has already beenproved during landings and at speeds up to 450 kt. A feature of production aircraft will be the dog-tooth leading-edge extensions which increase chord by two per cent and are drooped li per cent. Over the extension, the leading-edge radiusis increased and the dog-tooth is intended to avoid a reduction in trimmed CLmar at approach speeds by preventing part-spanvortex separation. Tunnel tests have indicated that 65 per cent of semi-soan is the optimum extent of the dog-tooth. The leading-edee discontinuity acts as a fence and the extension is small enough to have no effect on high-speed performance and stability. Thecamber is continued into the tip fairing to prevent tip-losses. To balance the larger wing and longer forward fuselage, theareas of both tailplane and fin have been increased in comparison with those of the Gnat fighter. The Lockheed aileron jacks have a negative feed-back mountingto improve stiffness and a fairly long valve travel to ensure stability. Should hydraulic pressure fail, the surface is operated
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